The vocal folds, because of their position in the airway, play a vital role in speech, swallowing, and breathing. In order to perform these functions normally, the laryngeal muscles must be able to open and close the folds. In addition, the folds must have the proper biomechanical properties to efficiently and effectively control the air stream when used, for voice production.
Each vocal fold is composed of a muscle covered by a ligament running the length of the vocal fold and a more superficial, free mucosal edge that vibrates during voice production. The tissues lying above the muscular body of the vocal fold, called the lamina propria, can be separated into discrete layers based on the concentration of elastin and collagen fibers and fiber orientation. The delicate arrangement of the extracellular matrix proteins within the lamina propria permits passive movement of a vocal cover over the body, resulting in the formation of a mucosal wave as air is passed through the glottis. Mobility of these tissue layers influences the fundamental vibration frequency of the vocal folds and directly impacts the voice.
Scar tissue may form in the vocal fold. This scar tissue can cause adhesion of the vocal fold epithelium to the vocal ligament or deeper tissues, effectively eliminating the gelatinous material of the superficial lamina propria at the scar location. Without the gelatinous layer of the lamina propria, a vocal fold is unable to generate a normal mucosal wave during phonation. Such a vocal fold is referred to as “dysphonic.”
Patients with dysphonia caused by vocal fold scarring are typically evaluated by indirect laryngoscopy and video stroboscopy, with particular attention paid to vocal fold mobility, glottic closure, and the presence, amplitude, and symmetry of the mucosal wave.
During speech, the mucosal wave is best observed by illuminating the vocal folds with evenly-spaced light pulses from a strobe light in a technique know as stroboscopy. If the pulsation frequency matches the fundamental vibration frequency of the vocal folds, then the folds will appear stationary even though they are vibrating. If, however, the strobe's pulsation frequency is slightly offset from that of the vocal folds, then the folds will appear to move in slow-motion. The visual appearance of the vocal fold mucosal wave when thus illuminated is a diagnostically important aspect of vocal fold assessment.
Injection of gelatinous material into the superficial lamina propria to treat vocal fold scarring is best performed when the patient is under general anesthesia. The targeted layers of vocal fold are thin and delicate, and must be stationary to ensure accurate injection without tissue trauma. The accuracy of the injection is increased when the surgeon has a direct, magnified view of the injection site through a glottiscope.
Surgeons who inject material into scarred vocal folds to restore pliability would benefit from feedback about how each injection made during the procedure affects vocal fold biomechanical properties. Unfortunately, glottiscope placement is invasive and requires general anesthesia. Under these circumstances, it is impractical to remove the glottiscope and awaken the patient between each intraoperative manipulation in order to ask the patient to phonate.